Disc memories, of the type widely used in data processing systems, carry encoded data on circular concentric tracks that can be situated on both faces of a disc. Typically, the recording tracks have a width of no more than a few dozen microns. The discs are driven at constant rotational velocity by a rotary output shaft of an electric motor. Data are written into, i.e., recorded, and read from magnetic discs by transducers, one or more of which is normally associated with one face of the disc. The disc passes in front of the transducer or transducers, causing the transducers to apply state transitions to the tracks or to enable the transducers to read state transitions from the tracks. In magnetic discs, the transitions are state changes of magnetic flux recorded on the disc by the transducers or read from the disc by the transducers. Most commonly used transducers are magnetic transducers generally comprising a magnetic circuit coupled to a winding and including an air gap. In the present specification and claims, the air gap is defined as part of a transducing means of the transducer. The air gap is symmetrical, with the center of symmetry being referred to as the center of the air gap and the center of the transducing means. Typically the air gap has a substantially rectangular shape, with a length much greater than the width thereof. The air gap longitudinal axis of symmetry is referred to in the present specification and claims as the "direction of the air gap."
To write or read data between the tracks on one face of the disc with the transducers, it is necessary to move the transducer or transducers associated with a particular disc face parallel to the face. To this end, the transducer is an integral part of a supporting arm held by a positioning system or actuator. In the prior art, linear actuators are commonly employed to translate the transducer along a radius of the disc face with which a particular transducer is associated. The transducer air gap subsists along the radius over which the transducer moves. In one type of magnetic disc, a data sequence is represented by a succession of small magnetic areas distributed along the length of each track. The magnetic areas have either positive or negative magnetic flux fields of equal amplitude, or no magnetic flux field. The direction of the flux field is either parallel or perpendicular to the disc surface. The number of elementary positive magnetic flux areas is equal to the number of elementary negative magnetic flux areas.
A physical boundary between two adjacent elementary magnetic flux areas having opposite polarities in the same track defines a magnetic transition. Thus, a magnetic transition is the physical zone where a change occurs in the physical state between two elementary magnetic flux areas; the physical state is defined by the direction of the magnetic flux transition, i.e., from positive to negative or from negative to positive. More generally, for any type of data recording system, such as capacitive, optical, et cetera, a transition is defined as a physical zone where a change occurs in a physical state between two adjacent elementary areas of a track.
In magnetic recording, a data bit is defined by at least one magnetic transition. The value of a data bit depends on the type of transition, i.e., from positive to negative or negative to positive. Alternatively, the magnetic transition can be represented by the presence or absence of a transition or on the type of association of the transition with another transition or transitions; one particular example of the latter transition type is data bits formed by an association of two magnetic transitions of opposite polarity, as disclosed in French Pat. No. 2,439,435, commonly owned with the present invention, entitled "Method of Writing Information On A Magnetic Recording Medium", which is the equivalent of U.S. Pat. No. 4,354,208. In the system disclosed in the '435 French patent, the value of a data bit is a function of the presence or absence of a double magnetic transition.
Currently available magnetic discs and transducers enable the packing density of the disc, in both the radial and circumferential directions, to be extremely large. The radial density of a disc is the number of tracks per unit length measured along a radius of the disc. The longitudinal density of a track is the number of data bits per unit length of the track, measured along the track circumference. By providing extremely high packing densities, the same amount of data can be stored on a smaller number of discs and the size of the discs can be decreased.
In the discs disclosed in French Pat. No. 2,439,435, the data are distributed in adjacent circular sectors each having the same area. One face of the disc is generally divided into several tens of sectors, each of which is divided into two unequal areas. The larger area contains data to be processed by a data processing system by which the disc memory is a part. The smaller area of each sector contains track identifying information used by a device for positioning the magnetic transducers facing the disc tracks.
The smaller area of each sector is divided into several reference zones. Each track is associated with two adjacent reference zones. The boundary between two reference zones coincides with the circumference of a circle located in the center of a track associated with the two zones. Thereby, when a magnetic transducer reads data from a given track, it reads the data of two adjacent reference zones because the transducer is astride the adjacent reference zones. Preferably, the transducer is perfectly centered on the boundary between the two reference zones. Each reference zone contains plural magnetic transitions, each occupying a specific, predetermined position and having a serial number within the zone.
To read or write data recorded on the magnetic tracks of a disc face in the larger, data area of each sector with the highest possible degree of accuracy, it is necessary for the transducer to be positioned relative to each track with the greatest possible accuracy. Hence, it is necessary to write the track identifying information into the smaller sectors with maximum precision, as is achieved by using a special formatter device. The track identifying information is initially written by the formatter into the disc for the entire lifespan of the disc, typically prior to the disc being sold or sent to a user.
Increasing the radial and longitudinal data densities contained on magnetic discs has enabled the size of the discs to be considerably reduced. Reducing the size of the discs has also resulted in a reduction in the size of memory units including the discs. This has been accompanied by the use of rotary actuators in the disc memory units because such actuators require less electric power than a linear actuator and because they require a smaller volume. A rotary actuator includes mobile structure which rotates about an axis parallel to the magnetic disc rotation axis; the axis of the mobile structure is located beyond the periphery of the disc. The rotary actuator carries a supporting arm for the data writing transducer in such a manner that the transducer moves in an arc from a track at the periphery of a disc toward a track at the disc center. Typically, the axes of rotation of the disc and actuator are spaced approximately 80 millimeters apart and the disc has a 65 millimeter radius. In such an instance, the transducer moves through an arc having a length of approximately 20 millimeters and a radius of approximately 60 millimeters relative to the actuator rotation axis.
Typically small disc memory units include a removable disc or two discs, only one of which is removable. The removable disc is located in a cartridge having a substantially rectangular window through which pass two platforms, each of which includes a transducer associated with a face of the disc which is located in the cartridge. The maximum dimensions of the window are on the order of 20 millimeters by 5 millimeters, to minimize air turbulence within the cartridge, as well as dust particle contamination. In the idle state, when the disc memory is not functioning, the transducers and a bearing platform for them are located outside of the cartridge. The transducers are activated to the read or write position within the cartridge by introducing the bearing platforms therefor from outside of the cartridge into the cartridge interior through the window. The width and height of each platform are respectively approximately 19 millimeters and 2 millimeters. When the transducer reads data from or writes data into the center most disc track, the platform has a length inside the cartridge on the order of 30 millimeters.
Under the conditions thus defined by the stated dimensions of the window, platform rotary actuator and the position of the actuator relative to the disc, it is impossible to position the transducer so that the direction of the air gap thereof is the same as a tangent to the path described by the transducer, or in such a way that the air gap direction subtends a zero angle with the tangent to the transducer path. Instead, the air gap direction forms an angle .phi. with the transducer path tangent; the transducer path is assimilated to the path described by the air gap center of symmetry. The angle .phi. has a value ranging from several degrees to a maximum of approximately 20 degrees.
The inability to position the transducers in such a way that the direction of the air gap thereof is the same as the tangent of the path described by the transducer causes the information to be read with a relatively low degree of accuracy from the reference zones in all sectors of the disc. Inaccuracies in reading the information from the reference zones may result in inaccurately determining the position of a transducer reading or writing data from and into the disc tracks, thereby producing errors in the data written into and read from the disc.
To appreciate the reason for the inaccuracy in reading the information from the reference zones, consider the situation of transitions written into the reference zones in accordance with the method described in the previously mentioned patent. The transitions written into the reference zones are identified relative to a permanent angular reference indicator carried by the disc. The reference angular. indicator enables the time origin, i.e., reference angular position, and therefore the phase origin of the disc to be accurately determined. Because the angular rotational velocity of the disc is maintained extremely constant, the phase of a point around the disc relative to the permanent angular reference indicator is a linear time function. In currently available disc memories and formatters, the angular reference indicator can be provided by a special indicator, either as a magnetic transition or by an optical transition formed by mechanical means. The special reference angular indicator is established at the periphery of the disc, on a special track, referred to as a clock track. The angular reference indicator is detected by a transducer which is different from the transducer used to detect data and track location information on the previously mentioned tracks. This special transducer is frequently called a clock transducer, which may be either a magnetic or opto electronic transducer.
During each disc revolution, the specific instant when the special transducer detects the transition in the clock track defines a time origin and therefore a phase origin. From the phase origin a succession of times t.sub.00, t.sub.01, t.sub.02, et cetera is defined such that: t.sub.01 =t.sub.00 +1/N, t.sub.02 =t.sub.01 +1/N, et cetera, where 1/N is the period of one revolution of the disc and N is the number of disc revolutions per second. The intervals between t.sub.00, t.sub.01, t.sub.02 et cetera define, for each revolution, the time and phase origins and the time sequences relative to the time and phase origins; this is because the phase is computed as a multiple of an integer to an accuracy of 2.pi. radians, and the disc velocity is maintained absolutely constant. The specific position occupied by the disc radius which passes the special indicator at each of instances t.sub.00, t.sub.01, t.sub.02 et cetera, thus is referred to as an angular reference indicator. The position of a given transition in a reference zone on a given track of a given sector is defined by the time separation between the time origin t.sub.00, t.sub.01 t.sub.02 et cetera associated with the sector to the occurrence of the transition. The time separation depends on the zone where the transition is located and on the angular velocity of the disc as it passes in front of a write head. For each exact known period there corresponds an angular position of a radius passing through a transition to be written into the disc, relative to the angular reference indicator. This angular position is referred to as a transition phase. When all transitions of the same serial number from all of the reference zones of a particular sector are written by a transducer driven by a linear actuator, as described in the previously mentioned patent, all of the transitions have the same phase. This provides for the highest degree of accuracy in positioning the read-write transducer above the disc during use.
However, if the transitions of a reference zone of a magnetic disc are written with a transducer driven by a rotary actuator, as described supra, in such a manner that all transitions of the same serial number of the reference zones of one sector have the same phase, several adverse consequences occur. In particular, the transitions within the reference zone are shifted relative to each other, with the centers of the transitions being aligned along an arc similar to that described by the air gap center. Each of the transitions forms an angle .phi. with the tangent to the arc so that the adjacent transitions are substantially parallel to each other. This parallelism causes the information to be read from the reference zones with a relatively low accuracy, with the adverse effects previously mentioned.